| Client-server is a relationship in which one program (the client) requests a service or resource from another program (the server). At the turn of the last century, the label client-server was used to distinguish distributed computing by personal computers (PCs) from the monolithic, centralized computing model used by mainframes. Today, computer transactions in which the server fulfills a request made by a client are very common and the client-server model has become one of the central ideas of network computing. The client establishes a connection to the server over a local area network (LAN) or wide-area network (WAN), and once the server has fulfilled the client's request, the connection is terminated. Because multiple client programs share the services of the same server program, a special server called a daemon may be activated just to await client requests. Until recently, the majority of network traffic was between clients and servers, a traffic pattern known as north-south. Increasingly, however, the volume of east-west (server to server) traffic has grown as a result of virtualization and data center trends such as cloud computing and converged infrastructure. The change is reflected in the way Chief Security Officers (CSOs) and network administrators are moving from a centralized security model designed to protect the confidentiality, integrity and availability (CIO triad) of network data within a perimeter to a distributed security model that focuses more on controlling individual user access to services and data, and auditing their behavior to ensure compliance with policies and regulations.
Client-server protocols
Clients typically communicate with servers by using the TCP/IP protocol suite. TCP is a connection-oriented protocol, which means a connection is established and maintained until the application programs at each end have finished exchanging messages. It determines how to break application data into packets that networks can deliver, sends packets to and accepts packets from the network layer, manages flow control and handles retransmission of dropped or garbled packets as well as acknowledgment of all packets that arrive. In the Open Systems Interconnection (OSI) communication model, TCP covers parts of Layer 4, the Transport Layer, and parts of Layer 5, the Session Layer. In contrast, IP is a connectionless protocol, which means that there is no continuing connection between the endpoints that are communicating. Each packet that travels through the Internet is treated as an independent unit of data without any relation to any other unit of data. (The reason the packets do get put in the right order is because of TCP.) In the Open Systems Interconnection (OSI) communication model, IP is in layer 3, the Networking Layer.
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